As we have a lot of ways to embed content on this forum, maybe a quick "1 post" refresher course is in order.

First up, not technically for embedding: links must be embedded inbetween URL tags
otherwise they don't show up as links anymore, just as plain text

Code:
[url]www.si-community.com[/url]

...if you're using the subsilver2 board style, your link not turning orange is a dead giveaway that you forgot to use these tags...

Next: the following tags work by placing the whole URL inbetween them:

Code:
[img]URL[/img]
[wmv]URL[/wmv]
[flv]URL[/flv]

The following tags take only their respective ID's between the tags

Code:
[vim]vimeo ID[/vim]
the vimeo ID is the last part of the URL, after "vimeo.com/"

Code:
[youtube]youtube ID[/youtube]
the youtube ID is the last part of the URL, after "watch?v="


Code:
[flvprev]partial URL[/flvprev]

In the case of the flvprev tags, you take the URL, but strip off the ".flv" end...

Code:
[flash=width,height]URL[/flash]

In the case of flash you provide the whole URL, but you also provide the width and the height
of the flash file you want to embed in the flash tag after the "=" sign.
Code:
For example:[flash=200,200]URL[/flash]

Note: the old vimeo tags still work, but are considered deprecated...

The nudity tag: place your image between img-tags
or your image attachment inline and copy this inbetween the nudity tags, like this:
Important: make sure, the attachment is placed inline, otherwise it still shows up as a normal attachment.

The hideme tag: very similar to the nudity tag with a different text ("Hidden content").
place your image between img-tags
or your image attachment inline and copy this inbetween the hideme tags, like this:
Important: make sure, the attachment is placed inline, otherwise it still shows up as a normal attachment.

To resize external images use the rimg-tag, you specify the new width of the image as it should appear in the post, the height is resized proportionally
where you obviously replace the text "width" by a number specifying the number of pixels of the width you want.
Note: the rimg-tag doesn't work with attachments only with external images.

To "embed" a specific tweet: get yourself the number shown in the URL after "status/" and put it between tweet tags. For example:

Lastly: If you're in the post editor and you hover over one of the tag buttons,
a short line of help is displayed below the buttons...